Multiple Metal Resistance and Co-resistance in Acer pseudoplatanus L. (Sycamore) Callus Cultures

In vitro growth of Acer pseudoplatanus L. (sycamore) callus tissue derived from shoot-tip explants and screened on metal-enriched media was studied in an attempt to identify resistance traits which may explain the survival of trees at metal-contaminated sites. Copper and Cd-resistance traits were identified in cell lines originating from trees at a site with a relatively recent history of severe contamination by these metals, and Cd- and Zn-resistance were identified in cell lines originating from mature trees at a mining spoil site with a much longer history of exposure to elevated concentrations of these metals. In Zn-resistant cell lines, co-resistance to Ni was also found, even though this metal was not elevated at the study site. This is the first report of multiple resistance and co-resistance to metals occurring at the cellular level in trees. The mechanisms of the measured resistance traits remain unclear, although there was evidence of reduced Cu and Ni uptake by resistant cell lines. It is concluded that facultative adaptations allowing acclimation to metal stress may be particularly significant for survival of mature trees; induction of metal resistance probably occurs in vivo in trees at metal-contaminated sites.